The present invention relates generally to valve systems for controlling a flow of fluid through a fluid passageway and, more particularly, to a valve system having a valve actuated by a piezoelectric device to control the flow of fluid through the valve system.
Valve systems have been designed in the past having a valve actuated by a solenoid, piezoelectric stack or magnetorestrictive rod to control the flow of fluid through the valve system. The valve system may comprise a common rail fuel injector, electrohydraulic actuator system, electronically-controlled fuel injector, gasoline port injector, fluid metering valve, relief valve, reducing valve, direct valve or direct-injection gasoline injector by way of example.
However, in solenoid-controlled valve systems, it is often difficult to accurately control movement and positioning of the valve through the control signals applied to the solenoids. This is especially true when intermediate positioning of a solenoid-controlled valve between two opposite, fixed positions is desired. Solenoid-controlled valves, by their very nature, are susceptible to variability in their operation due to inductive delays, eddy currents, spring preloads, solenoid force characteristics and varying fluid flow forces. Each of these factors must be considered and accounted for in a solenoid-controlled valve system design. Moreover, the response time of solenoids limits the minimum possible dwell times between valve actuations and makes the valve system generally more susceptible to various sources of variability.
While solenoids provide large forces and have long strokes, solenoids do have certain drawbacks. For example, first, during actuation, current must be continuously supplied to the solenoid in order to maintain the solenoid in its energized position. Further, to overcome the inertia of the armature and provide faster response times, a solenoid is driven by a stepped current waveform. A very large current is initially provided to switch the solenoid; and after the solenoid has changed state, the drive current is stepped down to a minimum value required to hold the solenoid in that state. Thus, a relatively complex and high power current driver is required.
In addition to requiring a relatively complex and high current power source, the requirement of continuous current flow to maintain the solenoid at its energized position leads to heating of the solenoid. The existence of such a heat source, as well as the ability to properly dissipate the heat, is often of concern depending on the environment in which the solenoid is used.
Additionally, the force produced by a solenoid is dependent on the air gap between the armature and stator and is not easily controlled by the input signal. This makes the solenoid difficult to use as a proportional actuator. Large proportional solenoids are common, but they operate near or at the saturation point and are very inefficient. Small, relatively fast acting non-proportional solenoids may have response times defined by the armature displacement as fast as 350 microseconds. However, this response time can be a significant limitation in some applications that require high repetition valve actuation rates or closely spaced events. Further, it is known that there is a substantial delay between the start of the current signal and the start of the armature motion. This is due to the inductive delay between the voltage and magnetic flux required to exert force on the armature. In valve systems, such delays lead to variability.
Electroactive actuators such as piezoelectric stacks and magnetorestrictive rods eliminate the response time and proportionality shortcomings of the solenoid. The piezoelectric stacks, due to their capacitive behavior, offer the benefit of drawing no power during xe2x80x9chold inxe2x80x9d, where actuation is maintained for a long period of time. However, these actuators have shortcomings of their own. Piezoelectric stacks and magnetorestrictive actuators possess impressive force, but have very small stoke capabilities. The output of these actuators must then be mechanically or hydraulically amplified, limiting the response time and proportionality benefits that they offer. Because of their small strain capabilities, these actuators also tend to be large. Additionally, these actuators are unidirectional, i.e., they move in only one direction in response to a control signal. Therefore, any valve or mass moved by the actuator requires a return biasing force, such as by a return spring, to be applied to return the valve or mass to its original position. Often, the spring comprises a significant amount of the force required to move the valve or mass and represents another source of variability. Also, the beneficial response time of the actuator will have no impact on the return of the valve or mass, as it depends completely on the return spring.
Thus, the present invention is directed to overcoming one or more of the problems set forth above.
While the invention is described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
In accordance with still yet another embodiment of the present invention, a valve system includes a fluid chamber and a fluid passage communicating with the fluid chamber. A fluid aperture is disposed at the juncture of the fluid chamber and the fluid passage. A pre-stressed bender actuator is operable to act directly on the fluid aperture between a closed position for closing fluid communication between the fluid chamber and the fluid passage and an open position for opening fluid communication between the fluid chamber and the fluid passage.